Acoustic-wave acquisition apparatus

ABSTRACT

With a detector in which detection elements are placed in a spherical shape, a uniform resolution area is narrow. 
     An acoustic-wave acquisition apparatus of the present invention is equipped with a detector including a plurality of detection elements that receive acoustic waves from a subject, the receiving surfaces of at least some of the detection elements being at different angles. The apparatus includes a scanning unit configured to move at least one of the subject and the detector to change the relative position of the subject and a highest-resolution area determined depending on the placement of the detection elements.

TECHNICAL FIELD

The present invention relates to an acoustic-wave acquisition apparatus.

BACKGROUND ART

General ultrasonic diagnostic apparatuses can acquire information insidea living organism by transmitting ultrasonic waves and receiving theultrasonic waves reflected inside the living organism. This allows adiseased part, such as cancer, to be detected. Furthermore, imaging ofphysiological information, that is, functional information, of a livingorganism attracts attention to improve the detection efficiency.Photoacoustic tomography (PAT) that uses light and ultrasonic waves hasbeen proposed as means for imaging functional information.

The photoacoustic tomography is a technology for imaging internaltissue, which serves as the source of acoustic waves, using thephotoacoustic effect in which acoustic waves (typically ultrasonicwaves) are generated by applying pulsed light generated from a lightsource to a subject and absorbing the light that has propagated anddiffused in the subject. Changes in the received acoustic waves withtime are detected at a plurality of locations, and the acquired signalsare mathematically analyzed, that is, reconstructed, and informationconcerning optical characteristic values of the internal part of thesubject is visualized in three dimensions.

The resolution of a three-dimensional image obtained using thephotoacoustic tomography depends on the following factors, depending onthe placement of acoustic detection elements. If a plurality of acousticdetection elements are placed on a planar surface, a resolution in adirection parallel to the placement planar surface (lateral resolution)depends on both the sizes of the receiving portions of the individualacoustic detection elements and frequencies that the acoustic detectionelements can detect, and a resolution in a direction perpendicular tothe placement planar surface (depth resolution) depends only onfrequencies that the acoustic detection elements can detect. Theresolution in the direction perpendicular to the placement planarsurface is higher than the resolution in the parallel direction becauseit is generally easier to increase the frequencies that can be detectedby the acoustic detection elements than decrease the size of thereceiving portions. In the case where a plurality of acoustic detectionelements are placed on a spherical surface, information in the depthwisedirection of all of the acoustic detection elements are superimposed,and thus, the lateral resolution is also equal to the depth resolution.That is, since the resolution in all directions depends only to thefrequencies, this placement offers high resolution. With intermediateplacement between planar placement and spherical placement in which aplurality of acoustic detection elements are placed on a plurality ofplanar surfaces provided at different angles, the resolution lessdepends on the sizes of the receiving portions of the acoustic detectionelements as the placement approaches from the planar placement to thespherical placement, thus allowing higher resolution to be achieved.

An example of an apparatus in which a plurality of acoustic detectionelements are placed on a spherical surface is disclosed in PTL 1. In PTL1, acoustic detection elements are placed in a spiral pattern on ahemispherical surface, and light irradiation and reception of acousticwaves using the acoustic detection elements are performed while thehemisphere is being rotated about a line connecting the poles of thehemisphere and the center of the sphere. Image reconstruction isperformed to obtain image data by using signals output from the acousticdetection elements that have received the acoustic waves.

CITATION LIST Patent Literature

PTL 1: U.S. Pat. No. 5,713,356

SUMMARY OF INVENTION Technical Problem

However, with the spherical placement of the acoustic detection elementsdisclosed in PTL 1, the resolution is the highest at the center of thesphere and decreases with a decreasing distance to the periphery,resulting in variations in resolution. In other words, since acousticwaves are incident at right angles on all of the acoustic detectionelements at the center, so that signals in the same phase enter at thesame time, the signals do not weaken. However, at portions other thanthe center, acoustic waves are diagonally incident on some of theacoustic detection elements, so that signals in the same phase enterwith a time lag. Thus, the weakening of the signals other than those atthe center is one of the causes of the variations in resolution.

Another cause is the directivity of the acoustic detection elements. Thetraveling direction of the acoustic waves is angled with respect to theacoustic detection elements, and the acoustic detection elements havedirectivity. Thus, the sensitivity is decreased when the travelingdirection is angled and is lost when the signal becomes weaker thannoise level. Thus, the resolution is decreased as the amount ofinformation decreases. For the planar surface type, when acousticdetection elements are placed on a planar surface that is sufficientlywider than the measuring range, a uniform resolution can be achieved inthe measuring range. For intermediate placement between planar placementand spherical placement in which a plurality of planar surfaces arearranged, a uniform-resolution range decreases gradually as theplacement shifts from the planar placement to the spherical placement.Thus, high resolution and the uniformity of resolution have a trade-offrelationship.

The present invention has been made on the basis of such problemrecognition. The present invention reduces variations in resolutiondepending on the location.

Solution to Problem

An acoustic-wave acquisition apparatus according to an aspect of thepresent invention is equipped with a detector including a plurality ofdetection elements that receive acoustic waves from a subject, thereceiving surfaces of at least some of the detection elements being atdifferent angles. The apparatus includes a scanning unit configured tomove at least one of the subject and the detector to change the relativeposition of the subject and a highest-resolution area determineddepending on the placement of the detection elements.

Advantageous Effects of Invention

The present invention can reduce variations in resolution depending onthe location as compared with the related art.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram showing the configuration of an apparatusaccording to a first embodiment of the present invention.

FIG. 2 is a schematic diagram illustrating the apparatus according tothe first embodiment of the present invention.

FIG. 3A is a diagram illustrating a scanning method according to thefirst embodiment of the present invention.

FIG. 3B is a diagram illustrating the scanning method according to thefirst embodiment of the present invention.

FIG. 3C is a diagram illustrating the scanning method according to thefirst embodiment of the present invention.

FIG. 3D is a diagram illustrating the scanning method according to thefirst embodiment of the present invention.

FIG. 4 is a flowchart showing the operation of the apparatus accordingto the first embodiment of the present invention.

FIG. 5A is a conceptual diagram illustrating the gradient of resolutionand the effect of scanning.

FIG. 5B is a conceptual diagram illustrating the gradient of resolutionand the effect of scanning.

FIG. 6A is a diagram illustrating a modification of the apparatusaccording to the first embodiment of the present invention.

FIG. 6B is a diagram illustrating a modification of the apparatusaccording to the first embodiment of the present invention.

FIG. 6C is a diagram illustrating a modification of the apparatusaccording to the first embodiment of the present invention.

FIG. 6D is a diagram illustrating a modification of the apparatusaccording to the first embodiment of the present invention.

FIG. 7 is a block diagram showing a configuration of an apparatusaccording to a second embodiment of the present invention.

FIG. 8 is a schematic diagram illustrating the apparatus according tothe second embodiment of the present invention.

FIG. 9 is a diagram illustrating a processing method of the apparatusaccording to the second embodiment of the present invention.

FIG. 10 is a block diagram showing another configuration of theapparatus according to the second embodiment of the present invention.

FIG. 11 is a flowchart showing the operation of the apparatus accordingto a third embodiment of the present invention.

DESCRIPTION OF EMBODIMENTS

The present invention is characterized in that variations in resolutionare reduced by moving at least one of a subject and an acoustic arraydetector in which a plurality of acoustic detection elements arearrayed. Embodiments of the present invention will be describedhereinbelow with reference to the drawings.

First Embodiment

A first embodiment is a basic embodiment of the present invention.First, the components of this embodiment will be described, and then amethod for placing acoustic detection elements and a method forscanning, which are features of the embodiment of the present invention,will be described. Thereafter, a method of this embodiment will bedescribed, and lastly conceivable variations will be described.

FIG. 1 is a block diagram showing the components of the firstembodiment. An acoustic-wave acquisition apparatus of this embodimentincludes a light source 1, a light irradiation unit 2, an acoustic arraydetector 5, a scanning unit 6, an electrical-signal processing unit 7, adata processing unit 8, and a display 9. The components and a subjectwill be described hereinbelow.

Light Source

The light source 1 is a device that generates pulsed light. To obtainhigh output, the light source 1 may be a laser or a light-emittingdiode. To generate photoacoustic waves effectively, the subject shouldbe irradiated with light for sufficiently short time depending on thethermal properties of the subject. If the subject is a living organism,it is preferable to set the pulse width of pulsed light generated fromthe light source 1 to tens of nanoseconds or less. Preferably, thewavelength of the pulsed light is in a near-infrared region called atherapeutic window, that is, about 700 nm to 1,200 nm. Light in thisregion can reach relatively deep in a living organism, thus allowinginformation of a deep part to be acquired. If measurement is restrictedto the surface of a living organism, visible light with a wavelengthfrom about 500 nm to 700 nm and light in the near-infrared region may beused. It is also preferable that the wavelength of the pulsed light havea high absorption coefficient for an observation target.

Light Irradiation Unit

The light irradiation unit 2 is a unit that guides pulsed lightgenerated from the light source 1 to a subject 3. Specific examplesinclude optical devices, such as an optical fiber, a lens, a mirror, anda diffuser. The shape and density of the pulsed light are sometimeschanged using these optical devices. The optical devices are not limitedto the above examples and may be any devices that satisfy the abovefunctions.

Subject

The subject 3 is the object to be measured. Specific examples include aliving organism, such as a breast, and, for adjustment of an apparatus,phantoms that simulate the acoustic characteristics and opticalcharacteristics of a living organism. Specifically, the acousticcharacteristics are a propagation speed and an attenuation rate ofacoustic waves, and the optical characteristics are a light absorptioncoefficient and a light scattering efficient. The subject 3 needstherein a light absorber having a high light absorption coefficient.Examples of the light absorber in a living organism include hemoglobin,water, melanin, collagen, and lipid. For the phantom, a substance thatimitates optical characteristics is sealed in as a light absorber. Inthe present invention, examples of the distribution of informationinside the subject 3, generated by receiving acoustic waves, include aninitial sound pressure distribution of acoustic waves generated due tolight irradiation, a light energy absorption density distributionderived from the initial sound pressure distribution, an absorptioncoefficient distribution, and a density distribution of substances thatconstitute the tissue. Examples of the substance density distributioninclude an oxygen saturation distribution and an oxidation-reductionhemoglobin density distribution.

Matching Layer

The matching layer 4 is an impedance matching material that fills aspace between the subject 3 and the acoustic array detector 5 toacoustically bond the subject 3 and the acoustic array detector 5. Amaterial thereof can be liquid that has acoustic impedance close tothose of the subject 3 and the acoustic detection elements and thatallows pulsed light to pass therethrough. Specific examples includewater, caster oil, and gel. Since the relative position of the subject 3and the acoustic array detector 5 changes, as will be described later,both the subject 3 and the acoustic array detector 5 may be placed in asolution that forms the matching layer 4.

Acoustic Array Detector

The acoustic array detector 5 is a detector including a plurality ofacoustic detection elements that convert acoustic waves into electricalsignals. The acoustic array detector 5 is placed on a surface in contactwith the solution that forms the matching layer 4 so as to surround thesubject 3. The acoustic detection elements that receive acoustic wavesfrom the subject 3 may have high sensitivity and a wide frequency band.Specific examples include acoustic detection elements using PZT, PVDF,cMUT, and a Fabry-perot interferometer. However, the acoustic detectionelements are not limited to the above examples and may be any acousticdetection elements that satisfy the above function.

Scanning Unit

The scanning unit 6 is a unit that scans (moves] the acoustic arraydetector 5 in three dimensions. In this embodiment, the subject 3 isfixed, and the acoustic array detector 5 is moved (scanned] using an XYZstage as the scanning unit 6 to change the relative position of thesubject 3 and the acoustic array detector 5. However, in the presentinvention, the relative position of the subject 3 and the acoustic arraydetector 5 need only be changed; the acoustic array detector 5 may befixed, and the subject 3 may be scanned. When the subject 3 is to bemoved, a configuration in which the subject 3 is moved by moving asupporting unit (not shown] that supports the subject 3 is conceivable.Alternatively, both the subject 3 and the acoustic array detector 5 maybe moved. The scanning may be continuously performed but may be repeatedin fixed steps. The scanning unit 6 can be an electrically driven stageequipped with a stepping motor or the like but may also be a manualstage. The scanning unit 6 is not limited to the above examples but maybe any scanning unit configured to move at least one of the subject 3and the acoustic array detector 5.

Scanning Control Unit

The scanning control unit 601 controls the scanning unit 6 to move thesubject 3 and the acoustic array detector 5 relative to each other.Specifically, the scanning control unit 601 determines the moving speedand direction of the scanning unit 6 and instructs the scanning unit 6of them. The scanning control unit 601 outputs information on the movingspeed and direction of the scanning unit 6 to the data processing unit8.

Electrical-Signal Processing Unit

The electrical-signal processing unit 7 has the function of amplifyinganalog electrical signals (receiver signals) output from the acousticarray detector 5 and converts the analog signals to digital signals(digital receiver signals). To efficiently obtain data, theelectrical-signal processing unit 7 may have the same number ofanalog-digital converters (ADC) as that of the acoustic detectionelements of the acoustic array detector 5; however, one ADC may beconnected by turns.

Data Processing Unit

The data processing unit 8 generates image data (image reconstruction)by processing the digital signals obtained by the electrical-signalprocessing unit 7. Specific examples of the data processing unit 8include a computer and an electrical circuit. Examples of imagereconstruction include Fourier transformation, universal backprojection, filtered back projection, and iterative reconstruction. Thepresent invention may use any image reconstruction.

Display

The display 9 displays image data created by the data processing unit 8as an image. Specific examples include a liquid crystal display and anorganic EL display. The display 9 may be separated from theacoustic-wave acquisition apparatus of the present invention.

Next, a method for placing a plurality of acoustic detection elements501 and a method for scanning the acoustic array detector 5, which arefeatures of the present invention, will be described. The placementmethod according to an embodiment of the present invention will bedescribed using FIG. 2. The acoustic detection elements 501 are fixed toa container whose inner wall (subject 2 side) is hemispherical, and thereceiving surfaces thereof face the center of the hemisphere. In thecase of the placement of FIG. 2, the resolution of an image acquiredusing universal back projection is highest at the center of thehemisphere and decreases with an increasing distance from the center.Even if the acoustic detection elements 501 are not placed on aspherical surface, the highest-resolution area depends uniquely on theplacement of the acoustic detection elements 501.

Here, in the present invention, a high-resolution area in the vicinityof the center that is the highest-resolution area is defined as ahigh-resolution area 301. The range of the high-resolution area 301depends on how much difference from the highest resolution is permitted.For example, if the acoustic detection elements 501 are placed in aspherical shape, the diameter r of the high-resolution area 301 isexpressed by Equation (1).

$\begin{matrix}\left\lbrack {{Math}.\mspace{14mu} 1} \right\rbrack & \; \\{r = {\frac{r_{0}}{\phi_{d}} \cdot \sqrt{\left( {R^{2} - R_{H}^{2}} \right)}}} & (1)\end{matrix}$

where R is an allowable resolution, R_(H) is the highest resolution, r₀is the diameter of a sphere on which the acoustic detection elements 501are placed, and r_(d) is the diameter of each of the acoustic detectionelements 501. By changing the relative position of the high-resolutionarea 301 and the subject 3 and performing reconstruction, the resolutionis uniformized. In the present invention, by changing the relativeposition of the highest-resolution area and the subject 3, the relativeposition of the high-resolution area and the subject 3 is eventuallychanged.

In the present invention, the sphere includes not only a perfect spherebut also an ellipsoid expressed by Equation (2) (a shape formed byexpanding an ellipse in three dimensions, whose surface is formed of aquadric surface).

$\begin{matrix}\left\lbrack {{Math}.\mspace{14mu} 2} \right\rbrack & \; \\{{\frac{x^{2}}{a^{2}} + \frac{y^{2}}{b^{2}} + \frac{z^{2}}{c^{2}}} = 1} & (2)\end{matrix}$

where a, b, and c are lengths of halves of the diameters in the x-axis,y-axis, and z-axis directions, respectively. An ellipsoid that satisfiesa=b=c is a perfect sphere. An ellipsoid in which two of a, b, and c areequal is a spheroid that is obtained by rotating an ellipse around theaxis of the ellipse. The sphere in the present invention also includes aspheroid. An ellipsoid is symmetric with respect to an x-y plane, a y-zplane, and a z-x plane, like a sphere.

In measurement, the inside of the hemispherical surface of the acousticarray detector 5 is filled with a solution serving as the matching layer4, and the subject 3 is placed in the solution. Laser light 201 isemitted so as to irradiate the subject 3 from the lower part (pole) ofthe hemispherical container. The acoustic array detector 5 is scanned bythe XYZ stage, which is the scanning unit 6, so that the positionrelative to the subject 3 is changed. Thus, the high-resolution area 301scans across the subject 3. Here, to obtain uniform resolution, thehigh-resolution area 301 may be scanned in the direction of nonuniformresolution, that is, the direction of gradient of the resolution. Theeffects thereof will be described later.

FIGS. 3A to 3D illustrate a specific scanning method. FIG. 3A shows aninitial position. Receiver signals are obtained while the entireacoustic array detector 5 is scanned in the direction of the arrow (tothe left in the plane of the drawing) using the XYZ stage. When theacoustic array detector 5 has reached the position in FIG. 3B, theentire acoustic array detector 5 is scanned downward in the plane of thedrawing to reach the state shown in FIG. 3C. Subsequently, the scanningand signal acquisition are performed until the positional relationshipin FIG. 3D is reached. After the operation is performed over the wholeone plane (in the X-Z plane), the acoustic array detector 5 is moved inthe depthwise direction (Y-direction) in the plane of the drawing, andthe scanning and signal acquisition are performed in the same way.

Next, a measuring method of this embodiment will be described withreference to FIG. 4. First, the light irradiation unit 2 irradiates thesubject 3 with pulsed light (S1). Acoustic waves excited in a lightabsorber in the subject 3 by the radiated pulsed light are received bythe acoustic detection elements 501 and are converted to receiversignals. The receiver signals are converted to digital signals by theelectrical-signal processing unit 7 (S2). At the same time, the dataprocessing unit 8 acquires scanning position information correspondingto the acquired digital signals from the scanning control unit 601 (S3).

Next, the scanning control unit 601 determines whether thehigh-resolution area 301 has finished scanning an entire measuring area(S4). The entire measuring area is not the entire subject 3 but may beany designated area. If the scanning has not been finished, the acousticarray detector 5 is scanned while the positional relationship among theacoustic detection elements 501 is fixed (S5), and the application ofthe pulsed light and the acquisition of signals of the acoustic wavesare repeated. “Fixing the positional relationship among the acousticdetection elements” means not moving the placement positions of theacoustic detection elements 501 on the acoustic array detector 5.

In S5, the scanning and the acquisition of the receiver signals may beperformed at regular intervals. In particular, the acoustic arraydetector 5 can be moved so that the pulsed light is applied at least onetime while the relative position of the high-resolution area 301 and thesubject 3 changes by a distance equal to the size (diameter) of thehigh-resolution area 301. This means that receiver signals are acquiredat least one time while the high-resolution area 301 moves a distanceequal to the size of the high-resolution area 301.

The smaller the distance scanned during the time from one lightirradiation to the next light irradiation, the more the resolution canbe uniformized. However, a small scanning distance (that is, a lowscanning speed) results in an increase in measurement time. Thus, thescanning speed and the receiver-signal acquisition time interval may beset as appropriate in consideration of desired resolution andmeasurement time.

The scanning is performed in three dimensions and in the direction ofthe gradient of the resolution. After the scanning across the entiremeasuring area is finished, the data processing unit 8 executes imagereconstruction based on the obtained digital signals and scanningposition information (S6). In the universal back projection used in theimage reconstruction, the acquired digital signals are subjected topre-processing, such as differentiation and noise filtering, and arethen subjected to inverse projection in which the signals are propagatedfrom the positions of the acoustic detection elements 501 in the reversedirection. This is performed on the acoustic array detector 5 at allscanning positions, and the propagated processed signals aresuperimposed. This processing allows a subject-information distribution,such as an absorption coefficient distribution, to be acquired as imagedata. Finally, the data processing unit 8 outputs the acquired imagedata to the display 9, and the display 9 displays an image (S7).

FIGS. 5A and 5B are schematic diagrams illustrating the effects ofuniformization of resolution in the scanning direction. The tonesexpress resolutions at individual locations, in which a dark toneexpresses high resolution, and a light tone expresses low resolution.FIG. 5A shows a resolution with a lateral gradient. If scanning isperformed in the direction of the gradient of resolution, the resolutionin the lateral direction is uniformized at high resolution except thearea at the right end, which is a scanning end portion.

On the other hand, FIG. 5B shows a resolution with a vertical gradient.If scanning is performed in the direction in which no resolutiongradient is present, the vertical resolution is not uniformized In thisembodiment, since the acoustic detection elements 501 are placed on aspherical surface, the resolution gradient is present in all directionsfrom the center of the sphere, and thus, scanning may be performed inany directions.

Next, conceivable variations (modifications of the first embodiment) ofthe present invention will be described. The scanning unit 6 need onlyperform three-dimensional scanning including not only linear scanningbut also rotational scanning. Specifically, the motion of rotating theacoustic array detector 5 about the optical axis of the laser light 201,shown in FIG. 2 and linear scanning may be combined. The scanning may beperformed so as to have a short path length.

To uniformize the resolution of the entire subject 3, it is preferablethat the hemi-spherical container serving as the acoustic array detector5 be twice or larger as the subject 3 so that the high-resolution area301 can scan the entire subject 3. In other words, in the case where aholding member (a subject holder 10 shown in FIG. 8, described later)for holding the subject 3 is used, it is preferable that the insidediameter of the acoustic array detector 5 (the diameter of thehemispherical surface on which the acoustic detection elements 501 areprovided) be twice or larger as the outside diameter of the holdingmember.

Furthermore, when three-dimensional scanning is performed, the volume ofthe subject 3 in the solution serving as the matching layer 4 ischanged. Therefore, an inlet through which the solution is poured and anoutlet through which the solution is discharged may be provided to keepthe level of the solution constant, thereby adjusting the amount of thesolution.

The acoustic detection elements 501 may be placed in a spherical shape;alternatively, they need not necessarily be placed in the sphericalshape but need only be placed on a curved surface or a planar surface toobtain a predetermined highest-resolution area. That is, in the presentinvention, the acoustic detection elements 501 need only be placed sothat the receiving surfaces face the subject 3, and the receivingsurfaces of at least some of the acoustic detection elements 501 are atdifferent angles. In other words, some of the acoustic detectionelements 501 may be placed in a concave shape with respect to thesubject 3 so that the receiving surfaces are at different angles. Ofcourse, the resolution less depends on the size of the receivingportions of the acoustic detection elements 501 as the placement of theacoustic detection elements comes close to a spherical shape.

FIGS. 6A to 6D illustrate examples of the placement of the acousticdetection elements 501 applicable to the present invention. In FIGS. 6Aand 6B, the acoustic detection elements 501 are placed along the curvedsurface of part of the spherical surface. Here, the curved surface inthe present invention includes not only a perfectly smooth curvedsurface but also a curved surface having partial irregularities. Theconfigurations shown in FIGS. 6A and 6B allow flexible placement of thelight irradiation unit 2 and so on. In FIG. 6C, the acoustic detectionelements 501 are placed along a curved surface that is not spherical. Insuch a case, the trade-off of the resolution and the uniformity ofresolution can be adjusted. In FIG. 6D, the acoustic detection elements501 are placed in two linear patterns (planar shapes). In such a case,since the acoustic detection elements 501 are placed in linear patternswith two different angles so as to surround the subject 3, a wideuniform resolution area can be provided, and the scanning step width canbe increased. Although FIGS. 6B and 6D show examples in which the numberof curved surfaces or planar surfaces on which the acoustic detectionelements 501 are placed is two, the acoustic detection elements 501 inthe present invention may be placed on surfaces more than that or, ofcourse, on one continuous surface. Any desired number of acousticdetection elements 501 may be provided.

In this embodiment, the configuration and processing method describedabove allows the resolution of an image acquired in the entire measuringarea to be higher than or equal to the high resolution and lower than orequal to the highest resolution, and the variations of the resolution tobe reduced, that is, a uniform resolution area to be increased.

Second Embodiment Signal-Attenuation Correction

In a second embodiment, a configuration for correcting receiver signalswill be described. When acoustic waves propagate in the subject 3 andthe matching layer 4, the intensity of the acoustic waves is attenuated.The distance of propagation of the generated acoustic waves in thesubject 3 and the distance of propagation in the matching layer 4 in thepath from the acoustic-wave generation position to the acousticdetection elements 501 depend on the scanning position of the acousticarray detector 5. In the case where the attenuation rates of the subject3 and the matching layer 4, which are formed of a living organism andwater or the like, respectively, differ, a correct contrast cannotsometimes be calculated. Thus, in this embodiment, a method forcorrecting different intensity attenuations will be described.

FIG. 7 shows the configuration of an acoustic-wave acquisition apparatusof the second embodiment. The configuration differs from that of thefirst embodiment in that the subject holder 10 is added as a holdingmember for holding the subject 3. The second embodiment also differs ina processing method in the data processing unit 8. Since the otherconfigurations are the same as those of the first embodiment,descriptions thereof will be omitted. As shown in FIG. 8, the subjectholder 10 holds the subject 3 and defines the shape of the subject 3.The subject holder 10 may be a thin, hard holding member whose acousticimpedance is close to that of the subject 3 or the matching layer 4.More preferably, the acoustic impedance is between those of the subject3 and the matching layer 4. A specific example is polymethylpentene.Preferably, the subject holder 10 has a thickness of 0.1 mm or more and5 mm or less.

A measuring method of the second embodiment differs from that of thefirst embodiment in the process of image reconstruction of the dataprocessing unit 8 (S6 in FIG. 4). In this embodiment, the boundarybetween the subject 3 and the matching layer 4 can be determined fromthe shape of the subject holder 10, and the area of the subject 3 andthe area of the matching layer 4 can be determined from acquired signalsby converting distance to time. Here, since the subject holder 10 issufficiently thin, propagation of acoustic waves in the subject holder10 is negligible.

For one acoustic detection element 501, the attenuation can generally beproperly corrected by dividing signals corresponding to individual areasby acoustic attenuation rates of the individual areas. However, as shownin FIG. 9, a signal obtained by one acoustic detection element 501 is asuperimposed signal of signals from a plurality of voxels. The time atthe boundary depends on the target voxel, so that the position of theboundary cannot be uniquely determined Thus, when a target voxel 1 is tobe reconstructed, a boundary 1 derived from the positional relationshipbetween the voxel 1 and the acoustic detection element 501 is set, andthe signal is divided by an attenuation rate corresponding to the areaon the basis of the boundary 1, thereby being corrected. Signals of theother acoustic detection elements 501 are also corrected in the sameway, are subjected to pre-processing, such as differentiation, and aresuperimposed to generate voxel data of the target voxel 1. Also for atarget voxel 2, a boundary 2 is set, and correction is performed on thebasis of the boundary 2.

With this embodiment, even if different boundaries are set for the samereceiver signal, the attenuation can be properly corrected bysuperimposing the receiver signals of the acoustic detection elements501. Thus, even if the acoustic attenuation rates of the subject 3 andthe matching layer 4 differ, a correct contrast can be calculated.

Furthermore, in this embodiment, the boundary between the subject 3 andthe matching layer 4 is determined from the shape of the subject holder10; instead, as shown in FIG. 10, a method of measuring the outer shapeof the subject 3 with a shape measurement unit 11 to obtain the positionof the boundary and correcting the contrast is conceivable. With thismethod, since the subject 3 is not contacted by the subject holder 10,the load on the subject 3 is reduced.

Third Embodiment Refraction Correction

A third embodiment is characterized by correcting signals inconsideration of the refraction of acoustic waves at the interface.Although the matching layer 4 may have an acoustic impedance close tothat of the subject 3, it is actually difficult to match the impedancescompletely. Accordingly, since an acoustic impedance is the product ofthe propagation speed and density of acoustic waves, the propagationspeeds of the acoustic waves in the matching layer 4 and the subject 3sometimes differ. In this case, the acoustic waves are refracted, thusdecreasing the resolution. Here, a method for correcting the refractionto improve the resolution will be described.

The configuration of the third embodiment is the same as theconfiguration of the second embodiment shown in FIG. 7, in which thesubject holder 10 is provided as a holding member for holding thesubject 3. The third embodiment also differs from the first and secondembodiments in a processing method in the data processing unit 8. Sincethe other configurations are the same as those of the first and secondembodiments, descriptions thereof will be omitted.

A measuring method of the second embodiment differs in the process ofimage reconstruction of the data processing unit 8 (S6 in FIG. 4). Inback projection in image reconstruction processing in whichpre-processed signals are propagated from the acoustic detectionelements 501 in the reverse direction and are superimposed, correctionis performed in consideration of refraction caused at the interfacebetween the subject 3 and the matching layer 4 during the backprojection. The acoustic velocities of the subject 3 and the matchinglayer 4 may be measured in advance because they are required to correctthe refraction.

As in the second embodiment, the boundary (interface) between thesubject 3 and the matching layer 4 can be determined from the shape ofthe subject holder 10, from which incident angles can be determinedFurthermore, since the acoustic velocities through the subject 3 and thematching layer 4 are known, refractive indexes can be derived from theacoustic velocity ratio. Since the refractive indexes and the angles ofincidence are known, refractive angles can be determined from Snell'slaw. Accordingly, in back projection of the processed signals, thesignals are propagated not straight but at the refractive anglescalculated at the boundary and are superimposed to generate image data.Also in this embodiment, since the subject holder 10 is sufficientlythin, propagation of acoustic waves in the subject holder 10 isnegligible.

With this embodiment, a decrease in resolution due to refraction causedby the difference between acoustic velocities can be corrected.Furthermore, as in the second embodiment, the boundary between thesubject 3 and the matching layer 4 can be measured using the shapemeasurement unit 11 instead of the subject holder 10.

Fourth Embodiment Real-Time Display

Although the reconstruction described in the first embodiment isperformed after all the signals have been obtained, measurement resultscannot be obtained till the end if the measurement time is long.Furthermore, if the measurement has failed, the time consumedunnecessarily. Thus, in the fourth embodiment, a method for displayingthe result in real time will be described.

Although the configuration of the fourth embodiment is the same as thatof the first embodiment shown in FIG. 1, processing in the dataprocessing unit 8 differs.

A measuring method of this embodiment will be described with referenceto FIG. 11. First, pulsed light is applied to the subject 3 (S1).Acoustic waves excited by the pulsed light are received by the acousticdetection elements 501, are converted to analog receiver signals, andare then converted to digital signals by the electrical-signalprocessing unit 7 (S2). The data processing unit 8 acquires scanningposition information corresponding to the acquired digital signals fromthe scanning control unit 601 (S3). The data processing unit 8reconstructs a high-resolution area 301 using the acquired signals (S8).The data processing unit 8 outputs image data reconstructed at aposition corresponding to the high-resolution area 301 at that time tothe display 9, and the display 9 displays an image (S9).

Next, the scanning control unit 601 determines whether thehigh-resolution area 301 has finished scanning the entire measuring area(S4). If the scanning has not been finished, the acoustic array detector5 is scanned (S5). Thereafter, steps S1, S2, S3, S8, and S9 arerepeated. Since the scanning step width is smaller than thehigh-resolution area 301, image display areas at the first measurementand the second measurement are superposed one on another. Thus, imagedata may be created by determining a mean value for the superposedareas. Repeating the processing allows an image to be displayed in realtime. However, since the number of signals for use in reconstruction issmall, so that the amount of information is small, the image quality islower than that of the first embodiment. Accordingly, after completionof scanning, the data processing unit 8 performs reconstruction usingall the signals (S6) to overwrite image data and displays it (S7).

The fourth embodiment allows measurement to be performed while theresults are being checked in real time.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application claims the benefit of Japanese Patent Application No.2011-027542, filed Feb. 10, 2011 and No. 2011-278895, filed Dec. 20,2011, which are hereby incorporated by reference herein in theirentirety.

REFERENCE SIGNS LIST

1 light source

2 light irradiation unit

3 subject

4 matching layer

5 acoustic array detector

6 scanning unit

7 electrical-signal processing unit

8 data processing unit

9 display

10 subject holder

11 shape measurement unit

1. An acoustic-wave acquisition apparatus equipped with a detectorincluding a plurality of detection elements that receive acoustic wavesfrom a subject, the receiving surfaces of at least some of the detectionelements being at different angles, the apparatus comprising a scanningunit configured to move at least one of the subject and the detector tochange the relative position of the subject and a highest-resolutionarea determined depending on the placement of the detection elements. 2.The acoustic-wave acquisition apparatus according to claim 1, whereinthe detector is a container whose subject-side surface is a sphericalsurface, and the detection elements are placed on the spherical surface.3. The acoustic-wave acquisition apparatus according to claim 1, whereinthe scanning unit moves at least one of the subject and the detector inthree dimensions.
 4. The acoustic-wave acquisition apparatus accordingto claim 1, further comprising a control unit configured to control themovement of the scanning unit.
 5. The acoustic-wave acquisitionapparatus according to claim 1, further comprising a holding memberconfigured to hold the subject.
 6. The acoustic-wave acquisitionapparatus according to claim 1, further comprising a shape measurementunit configured to measure the outer shape of the subject.
 7. Theacoustic-wave acquisition apparatus according to claim 5, wherein thedetection elements are placed in a hemispherical shape, and the diameterof the hemispherical shape including the detection elements is twice ormore of the outside diameter of the holding member.
 8. The acoustic-waveacquisition apparatus according to claim 2, wherein the control unitcontrols the scanning unit so as to move at least one of the subject andthe detector in the direction of the gradient of the resolution.
 9. Theacoustic-wave acquisition apparatus according claim 2, furthercomprising a light source configured to generate pulsed light, whereinthe acoustic waves are generated at the subject when the subject isirradiated with the pulsed light generated by the light source, whereinthe control unit controls the movement of the scanning unit so that thepulsed light is applied to the subject at least one time while therelative position changes by a distance equal to the size of thehigh-resolution area.
 10. The acoustic-wave acquisition apparatusaccording to claim 1, further comprising a processing unit configured tocreate a distribution of information inside the subject using receiversignals output from the detection elements, wherein the processing unitcreates the information distribution using a plurality of receiversignals that are obtained at individual locations by moving at least oneof the subject and the detector.